It's hilarious, since he was just messing with her.
It kinda started with his sincere irritation for her low self-esteem, he was saying it could be that way, but he got swept by messing with her halfway, since he just can't help it!
I don't know what the original FL personality was supposed to be, but if she was really kind-of-Cinderella, kind and nice and a bit meek like Emily, we might be looking on a different story altogether (I mean, not only reincarnated Dylan is different, but ML also). Because he clearly enjoys a challenge, and strong-willed and daring women (probably also people in general), she provokes him constantly and he can't help getting back at her but he also clearly enjoys the process.
He was scheming and wondering how she'll try to get out of the situation when he approached her and Emily at the ball, and it's the same in the last part in the park.